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Drying with Fixed-Bed Desiccants

IP.com Disclosure Number: IPCOM000182697D
Publication Date: 2009-May-05
Document File: 8 page(s) / 1M

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Page 1 of 8

ng ith Fixe

Desiccants

Fundamentals'

  When activated desiccant is brought into contact with a of high humidity, the pressure of the water vapor in the adsorbent tends to reach equilibrium with the of the water vapor in the surrounding gas. Hence the moisture content of the gas is de- creased.

 Physical adsorption is a continuous process. Ad- sorbed molecules break away from the adsorbent when they acquire sufficient energy. When the rate at which water molecules leave the adsorbent is equal to the rate at which they are being adsorbed on the surface, an equilibrium is established and no further adsorp- tion takes place.

  The amount of water adsorbed at equilibrium in- creases with higher partial pressure of the water in the gas, and decreases with temperatures. The desiccant is said to be saturated a given set of when equilibrium and the of water adsorbed at this is known as tht! equilibrium capacity or static capacity.

The most convenient way to pra'!ent equilibrium

Reprinted from CHEMICAL ENGINEERING, Aug. 31, 1964

 Copyright 1964 by McGraw-Hili, Ine. 330 West 42 St., New York. N. Y. 10036

and distributed bv The M. W. Ketlog", (';omnRnv. a riivi!;;inn of PnllmAn Tnf".

[This page contains 1 picture or other non-text object]

Page 2 of 8

FIXED-BED DESICCANTS • • .

data is to plot the amount of water adsorbed the desiccant relative the variables of n; ..

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to cover

curves as parameters to completely express the data, A series of equilibrium curves for four popular desic- cants--silica (Davison Grade , molecular sieves

maIly only a fraction of equilibrium Equilibrium curves establish value for capacity,
whether he should

inlet gas is

Operation

What takes from

it

                           of moisture- sensitive materials and drying of air between double windows, and also for certain closed dynamic systems such as refrigerant dryers. They are of limited value, however, to the engineer designing a large-scale, fixed- bed dehydration unit. In a dynamic system, the flowing gas and the desiccant particles that it contacts in its passage through the tower can only approach static equilibrium capacities under very limited operating conditions. Hence, useful design capacities are nor-

UrI",,,,,e!! by adsorption mechanism

an operating desiccant tower. At the start of opera- tion with a freshly reactivated desiccant, the moisture or other adsorbate is removed from the flowing gas stream at the inlet section of the bed (to some equili- brium level). The distance required for this removal is the depth of bed known as the adsorption zone. This zone moves through the bed at a uniform rate de- pending on the operating conditions, and the effluent moiBture from the bed will be at a low value until the adsorption zone reaches the exit end of the bed. A small amount of additional adsorption occurs at the

curves ",h,D""rino

after nermal reactivation...